Synchronous motors, including line start, interior permanent magnet (LSIPM) motors, are typically very efficient. A LSIPM motor will produce torque to accelerate from zero speed when started across the line, and then operate as a synchronous motor with no rotor cage losses once fully up to synchronous speed. However, synchronous motors have limited capability to pull into synchronism loads that have a high torque or high inertia. For certain applications, it is necessary for a LSIPM to demonstrate satisfactory starting performance in addition to the steady-state performance. For a LSIPM motor, this includes more than just meeting rated starting current and starting torque during the asynchronous period of acceleration as would be the case for an induction motor. The LSIPM motor must also be able to pull a load into synchronism and achieve normal steady state operation. Both load torque and load inertia are considerations whether a specific LSIPM motor will be able to successfully start and synchronize a load. Accordingly, the benefits in efficiency gains and energy savings ordinarily associated with synchronous motors are not typically achieved in applications having loads with high inertia and/or high torque characteristics. In the past, an inverter has been used with synchronous motors in such applications to power the motor during starting. However, an inverter adds substantial costs and degrades system efficiency.
To achieve the steady state benefits of efficiency provided by synchronous motors, and reduce limitations during start-up, rotor end rings and rotor bars may be designed to improve the ability of a motor to synchronize loads with higher torque and/or inertia requirements compared with similar motors having conventional end ring and rotor bar designs. The rotor end rings and rotor bars may be configured to reduce full load asynchronous slip by decreasing rotor resistance during start-up. While a decrease in rotor resistance may theoretically be achieved using induction motor principles (i.e., by increasing the total cross sectional area of the rotor bars forming the starting cage), increasing the area of the rotor bars has a negative impact on the starting and full load operating performance of the motor. For instance, increased rotor bar area results in increased flux density in the rotor and lower power factor, higher current, and more losses at full load, and higher locked rotor current at starting.